[PATCH v4] init: Disable defaults if init= fails

[Andy Lutomirski]

If a user puts init=/whatever on the command line and /whatever
can't be run, then the kernel will try a few default options before
giving up.  If init=/whatever came from a bootloader prompt, then
this is unexpected but probably harmless.  On the other hand, if it
comes from a script (e.g. a tool like virtme or perhaps a future
kselftest script), then the fallbacks are likely to exist, but
they'll do the wrong thing.  For example, they might unexpectedly
invoke systemd.

This makes a failure to run the specified init= process be fatal.

As a temporary measure, users can set CONFIG_INIT_FALLBACK=y to
preserve the old behavior.  If no one speaks up, we can remove that
option entirely after a release or two.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---

Changes from v3:
 - Get rid of the strictinit option.  Now the new behavior is the default
   unless CONFIG_INIT_FALLBACK=y (Rob Landley)

Changes from v2:
 - Improve docs further, to leave the door open to giving strictinit
   some sensible semantics if init= is not set.
 - Improve error output in the failure case (Shuah Khan).

Changes from v1:
 - Add missing "if" to the docs (Randy Dunlap)

 init/Kconfig | 11 +++++++++++
 init/main.c  |  7 ++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index e84c6423a2e5..063029a1556f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1299,6 +1299,17 @@ source "usr/Kconfig"
 
 endif
 
+config INIT_FALLBACK
+	bool "Fall back to defaults if init= parameter is bad"
+	help
+	  If enabled, the kernel will try the default init binaries if an
+	  explicit request from the init= parameter fails.
+
+	  This is a temporary measure to allow broken configurations
+	  to continue to boot.
+
+	  If unsure, say N.
+
 config CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size"
 	help
diff --git a/init/main.c b/init/main.c
index bb1aed928f21..d515b7b01ecd 100644
--- a/init/main.c
+++ b/init/main.c
@@ -960,8 +960,13 @@ static int __ref kernel_init(void *unused)
 		ret = run_init_process(execute_command);
 		if (!ret)
 			return 0;
+#ifndef CONFIG_INIT_FALLBACK
+		panic("Requested init %s failed (error %d) and strictinit was set.",
+		      execute_command, ret);
+#else
 		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
-			execute_command, ret);
+		       execute_command, ret);
+#endif
 	}
 	if (!try_to_run_init_process("/sbin/init") ||
 	    !try_to_run_init_process("/etc/init") ||
-- 
1.9.3

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